This disclosure relates generally to timing synchronization. More particularly, this disclosure may relate to systems and methods of synchronizing remote clocks with sub-picosecond precision, and distributing such precision across remote devices and systems.
Early clocks utilized the constant movement of an object to mark the passage of time. Such movement could include the motion of the sun across the sky (or shadows formed from the same), or the flow of water or sand at a relatively constant rate. Modern clocks, however, are the product of two components: an oscillator and a time interval counter. The oscillator precisely demarcates intervals of time, while the time interval counter advances the interval of time based on the completion of a determined number of oscillations. Although the vibration of quartz crystals utilized in modern clocks for everyday use permits accuracy to within a minute each year, there are situations where even greater accuracy becomes important.
Atomic clocks, which rely on oscillation between energy levels of atoms when probed by microwaves, have greatly advanced timekeeping in the past fifty years. For example, the standard definition of a second utilizes probing the oscillation of cesium-133 with microwaves at a frequency of approximately 9.192×109 Hz. While the first atomic clock, which utilized a beam of hot cesium atoms, was stable to about one part in 1010, further developments such as progressing to a fountain of cold cesium atoms has allowed an average stability of about one part in 1013. However, the greater stability provided by cooling the cesium atoms is limited by the potential for collisions between the atoms in the fountain, which may shift the frequency of the atomic transition. From fountain clocks, the state of the art has progressed even further. By utilizing light as opposed to microwaves, optical clocks allow a much greater frequency for measuring the atomic transitions. For example, instead of the 1010 Hz frequency of microwaves, light has a frequency of about 1015 Hz, allowing potentially greater clock stability.
The distribution and synchronization of the precise timing signals of advanced clocks, such as optical clocks, is increasingly important when dealing with communication and data transfer of remote elements. For example, satellite networks, electrical grids, differing subsystems of airplanes, and scientific laboratories across the globe, may desire highly synchronized master clocks, or the ability to receive precision timing from a master clock. As one non-limiting example, synchronized clocks are utilized when dealing with satellite communication, both in the context of satellite to satellite, as well as satellite to ground. The immense speed of orbiting bodies adds to the desirability of knowing exactly when particular actions should take place in a first system, so as to be harmonious with actions in a remote second system. In some contexts, precision timing may relate to knowing when a particular system, such as a satellite, is within communications range for a transmitter, while in other contexts, this may relate to delaying communications for synchronous data transfers, such as between satellites in a constellation or array, or between satellites and the ground. Effects of synchronization error include limiting the navigation accuracy of global positioning systems (GPS), and less precise data correlation between different sources, and instabilities in electrical grids.
What are needed are systems and methods that permit enhanced distribution of precise signals from clock systems, and enhanced synchronization between clock systems.